<html>

<head>
    <meta name="viewport"
          content="width=device-width, initial-scale=1, maximum-scale=1, minimum-scale=1, user-scalable=no">
</head>

<body>
    <style>
        .box {
            width: 5rem;
            height: 2rem;
            background-color: skyblue;
        }
    </style>

    <div class="box"></div>

    <script src="https://cdn.jsdelivr.net/npm/amfe-flexible@2.2.1/index.min.js"></script>
</body>

</html>